Build a new kernel, e.g. using build.sh. It will tell you where the ELF version of the kernel is, e.g.
...
Kernels built from RPI2:
/Users/feyrer/work/NetBSD/cvs/src-current/obj.evbarm-Darwin-XXX/sys/arch/evbarm/compile/RPI2/netbsd
...

Besides the "netbsd" kernel in ELF format, there is also a "netbsd.bin" kernel that is in a format that the Raspberry can boot.